This paper is concerned with the effects of boundary conditions on the large-amplitude free vi-brations of Timoshenko beams. The effects of nonlinear terms on the frequency of Timoshenko beams with simply supported ends (supported-supported, SS), clamped ends (clamped-clamped, CC) and one end simply supported and the other end clamped (clamped-supported, CS) are discussed in detail. Given a spe-cific vibration amplitude, the change of nonlinear frequency according to the effects of boundary conditions is always in the following descending order: SS, CS, and CC. It is found that the slenderness ratio has a significant influence on the nonlinear frequency. For slender beams, the nonlinear effects of bending curva-ture and shear strain are negligible regardless of the boundary conditions. For short beams and especially for those of large amplitude vibrations, however, the nonlinear effects of bending curvature and shear strain become noticeable in the following ascending order: SS, CS, and CC. This paper is concerned with the effects of boundary conditions on the large-amplitude free vi-brations of Timoshenko beams. The effects of nonlinear terms on the frequency of Timoshenko beams with simply supported ends (supported-supported, SS), clamped ends (clamped -clamped, CC) and one end simply supported and the other end clamped (clamped-supported, CS) are discussed in detail. Given a spe-cific vibration amplitude, the change of nonlinear frequency according to the effects of boundary conditions is always in the following descending order: SS, CS, and CC. It is found that the slenderness ratio has a significant influence on the nonlinear frequency. For slender beams, the nonlinear effects of bending curva-ture and shear strain are negligible regardless of the boundary conditions . For short beams and especially for those of large amplitude vibrations, however, the nonlinear effects of bending curvature and shear strains become noticeable in the following ascending order: SS, CS, an d CC.